fix(frontend): refit différé des cellules terminal après mutation de layout (#61)

Le ResizeObserver de TerminalView ne déclenche pas toujours un événement
utile quand une cellule voisine apparaît/disparaît (split/merge), forçant
l'utilisateur à redimensionner la fenêtre pour rafraîchir le scaling xterm.
useLayout expose un layoutVersion bumpé à chaque commit d'arbre (chargement
initial inclus), relayé par LayoutGrid comme refitSignal à chaque
TerminalView survivant pour déclencher fit.fit() sans rouvrir le PTY.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/layout/LayoutGrid.tsx

@@ -917,6 +917,7 @@ function LeafView({
           onSessionId={(sid) => void vm.setSession(id, sid)}
           agentMode={agentId != null}
           portal={agentId != null ? portal : undefined}
+          refitSignal={vm.layoutVersion}
         />
         {/* Write-portal overlay (ARCHITECTURE §20.3 step b/e): while a delegation
             is being injected into the agent's PTY, a grey veil with a centred

frontend/src/features/layout/useLayout.ts

@@ -23,6 +23,15 @@ import { leaves, splitOp } from "./layout";
 export interface LayoutViewModel {
   /** The current layout tree, or `null` until loaded. */
   layout: LayoutTree | null;
+  /**
+   * Bumped every time a mutation (split/merge/resize/move/…) commits a new
+   * tree, INCLUDING the initial load (ticket #61). `LayoutGrid` forwards it to
+   * every mounted `TerminalView` as `refitSignal` so surviving terminals refit
+   * after a structural change (split/merge) instead of waiting for a manual
+   * window resize — their own `ResizeObserver` doesn't always fire a useful
+   * event when a SIBLING cell appears or disappears.
+   */
+  layoutVersion: number;
   /** Last error message, or `null`. */
   error: string | null;
   /** Whether a request is in flight. */
@@ -65,9 +74,18 @@ export function useLayout(
 ): LayoutViewModel {
   const { layout: gateway, terminal } = useGateways();
   const [layout, setLayout] = useState<LayoutTree | null>(null);
+  const [layoutVersion, setLayoutVersion] = useState(0);
   const [error, setError] = useState<string | null>(null);
   const [busy, setBusy] = useState(false);
 
+  // Commits a freshly-loaded/mutated tree AND bumps the refit signal (ticket
+  // #61) in one place, so every caller below (initial load, `mutate`,
+  // `mutateChain`) stays a one-liner instead of repeating the pair.
+  const applyTree = useCallback((tree: LayoutTree) => {
+    setLayout(tree);
+    setLayoutVersion((v) => v + 1);
+  }, []);
+
   useEffect(() => {
     // No project, or a DI subset without the layout gateway (e.g. a focused
     // test) → render nothing rather than crash the tab (mirrors TerminalView).
@@ -80,7 +98,7 @@ export function useLayout(
     gateway
       .loadLayout(projectId, layoutId)
       .then((tree) => {
-        if (!cancelled) setLayout(tree);
+        if (!cancelled) applyTree(tree);
       })
       .catch((e: unknown) => {
         if (!cancelled) setError(describe(e));
@@ -91,7 +109,7 @@ export function useLayout(
     return () => {
       cancelled = true;
     };
-  }, [gateway, projectId, layoutId]);
+  }, [gateway, projectId, layoutId, applyTree]);
 
   const mutate = useCallback(
     async (operation: LayoutOperation) => {
@@ -99,14 +117,14 @@ export function useLayout(
       setBusy(true);
       setError(null);
       try {
-        setLayout(await gateway.mutateLayout(projectId, operation, layoutId));
+        applyTree(await gateway.mutateLayout(projectId, operation, layoutId));
       } catch (e) {
         setError(describe(e));
       } finally {
         setBusy(false);
       }
     },
-    [gateway, projectId, layoutId],
+    [gateway, projectId, layoutId, applyTree],
   );
 
   // Applies several operations in series, persisting each, but commits a single
@@ -124,14 +142,14 @@ export function useLayout(
         for (const op of operations) {
           tree = await gateway.mutateLayout(projectId, op, layoutId);
         }
-        if (tree) setLayout(tree);
+        if (tree) applyTree(tree);
       } catch (e) {
         setError(describe(e));
       } finally {
         setBusy(false);
       }
     },
-    [gateway, projectId, layoutId],
+    [gateway, projectId, layoutId, applyTree],
   );
 
   const split = useCallback(
@@ -230,6 +248,7 @@ export function useLayout(
 
   return {
     layout,
+    layoutVersion,
     error,
     busy,
     split,